Knife sharpener



FIG. 1 is a front top and right side perspective view of a knife sharpener showing my new design. The broken lines are for illustrative purposes only and form no part of the claimed design;

FIG. 2 is a front elevational view;

FIG. 3 is a right side elevational view;

FIG. 4 is a bottom plan view;

FIG. 5 is a right elevational view; and,

FIG. 6 is a rear elevational view thereof. 

The ornamental design for an knife sharpener, as shown and described. 